The Diest Formation : a review of insights from the last decades

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The Diest Formation: a review of insights from the last decades

Rîk HOUTHUYS^1*, RîekO ADRiAeNS², S^TijN GOOLAeRTS³, PîeT LAGA⁴, S^TePHeN LOUWYe⁵, jÔHAN MATTHijS⁶, NÔëL VANDeNBeRGHe⁷ & jÂSPeR VeRHAeGeN⁸

1 Geoconsultant, Halle, Belgium; rik.houthuys@telenet.be.

2 Qmineral, Heverlee-Leuven, Belgium; radriaens@qmineral.com.

3 OD Earth & History of Life and Scientific Heritage Service, Royal Belgian Institute of Natural Sciences, Vautierstaat 29, B-1000 Brussels, Belgium; stijn.goolaerts@naturalsciences.be.

4 Geological Survey of Belgium, Brussels, Belgium; piet.laga@skynet.be.

5 Geology, UGent, Belgium; stephen.louwye@ugent.be.

6 VITO, Mol, Belgium; johan.matthijs@vito.be.

7 Dept. Earth and Environmental Sciences, KU Leuven, Belgium; noel.vandenberghe@kuleuven.be.

8 VPO, Planning Bureau for the Environment and Spatial Development, Department of Environment, Flemish Government, Koning Albert II-laan 20, 1000 Brussels, Belgium; jasper.verhaegen@vlaanderen.be.

*corresponding author.

ABSTRACT. Research conducted since the 1960s on the upper Miocene Diest Formation in NE Belgium is reviewed and integrated.

Their lithology unites the deposits of the glauconiferous Diest Sand in one formation, though biozones and internal sedimentary structures strongly suggest the formation may agglomerate the deposits of two separate, successive sedimentary cycles.

The lowermost cycle is thought to have deposited the “Hageland Diest sand” during the early or middle Tortonian. It contains the Diest Sand in the main outcrop area in Hageland, Zuiderkempen and central Limburg, and probably also the Deurne Member near the city of Antwerpen. It furthermore includes the lower part of the Dessel Member in the central Kempen and in the Belgian part of the Roer Valley Graben (RVG). The Hageland Diest cycle represents the infill of a large tidal inlet tributary to the southern North Sea bight, then situated over the southern Netherlands and the Lower Rhine embayment. The Hageland Diest sand has the composition of a marine deposit, yet the confined area of occurrence and the presence of tens of metres deep incisions at the base, set it apart. The confinement of the embayment, strong tides and a steady supply of coastal-marine sand are invoked as the main driving forces that resulted in the distinctive geometry and internal architecture of the unit.

The upper cycle is associated with the “Kempen Diest sand”, which is found in the subsurface of the RVG and the Noorderkempen.

It has a late Tortonian to earliest Messinian age with progressively younger ages occurring to the NW. It encompasses the upper part of the Dessel Member and the overlying, coarser Diest Sand, and correlates to most or all of the thickly developed Diessen Formation in The Netherlands. It is the deposit of a prograding marine delta, containing both marine components and continental components fed by the palaeo-Meuse/Rhine river mouths. Accommodation space kept increasing during deposition, due to subsidence of the deposition area, especially inside the RVG but also in the Noorderkempen.

Although there is a fair consensus on the above, many concrete points about the geometry and depositional history of the Diest Formation and even a definitive decision on its single or dual character remain to be sorted out.

In addition, this review excludes the Flemish Hills sand and the Gruitrode Member from the Diest Formation.

KEYWORDS: Upper Miocene, Tortonian, confined embayment, deltaic progradation, depositional model, lithologic provenance, glauconiferous quartz sand.

https://doi.org/10.20341/gb.2020.012 1. Introduction and aim of this review

The Diest Formation is an important Neogene deposit in NE Belgium (Fig. 1a). It consists of greyish-green to brownish-green, fine to coarse grained, locally clayey, glauconiferous sand. In outcrop in the Hageland and Zuiderkempen areas (Fig. 1a, b), the sand is locally iron-cemented. The formation constitutes the interior of the longitudinal, parallel Hageland Hills around Leuven and Diest (Gullentops, 1957; Houthuys & Matthijs, 2018). More to the north, in the Kempen (Campine) subsurface, it is found as a tens of metres thick sand deposit that hosts an important aquifer.

The main lithological composition and appearance in both regions are similar, though certain aspects of their lithology and style and time of deposition are different (Vandenberghe et al., 2014). Below the Hageland outcrop area and its NE subsurface extension, an important incised depression is present at the base of the Diest Formation. To the west, some outliers on the Flemish hills, a long row of hills near the Flanders-Wallonia boundary, from Flobecq to Cassel in northern France (Fig. 1a), were traditionally assigned to the Diest Formation although doubt about this designation has always existed (review in Houthuys, 2014).

Scientific contributions of the past sixty years addressed the precise age of deposition, the internal variation, the depositional conditions, the extent and the relationship to outliers. They significantly adjusted the views held by the 1960s. This article summarizes and integrates the contributions and describes the present-day views of the Diest Formation. Hyperlinks throughout this text refer to the Flemish Authorities’ borehole and outcrop database “DOV” (Databank Ondergrond Vlaanderen) that holds locations, descriptions, interpretations, logs etc. (Table 1) (De Nil et al., 2020, this volume).

2. History and definition of the Diest Formation

The Diest Formation has been recognized as a lithostratigraphic unit in the earliest geological publications. The name “Diestien”, after the town of Diest in the NE corner of the province of Vlaams-Brabant (Fig. 1b), was formally introduced by Dumont (1839). He divided the Belgian Cenozoic rocks into six systems, one of which was the “système Diestien”. It included “sable glauconifère, sable ferrugineux et grès ferrugineux” occurring in the Hageland type area. It can be inferred from Dumont (1849) that he included the relatively coarse glauconiferous sand in the Kempen subsurface in this unit. Though trace fossils are abundant, the sand is mostly devoid of macrofossils, especially in the outcrop area (De Meuter & Laga, 1976). This fact is at the origin of some controversies about lateral correlations with outliers. At a small number of locations (Pellenberg, Bolderberg), silicified, reworked Bolderberg Formation fossils, as well as a mixture of Diest and Bolderberg Formation faunas, have been found in the base gravel (Tavernier & de Heinzelin, 1962). However, in a small area near Antwerpen, macroscopic calcareous fossils do occur in abundance in the local member called Deurne Sand (Figs 2 and 3, De Meuter & Laga, 1976); and in the Kempen subsurface, calcareous and organic walled microfossils are locally well preserved in the Dessel Member.

The Diest Formation was first included in the Pliocene series (1896 and 1929 geological maps) and later in the upper Miocene (de Heinzelin, 1955; Glibert & de Heinzelin, 1955a,b).

Foraminiferal biostratigraphical studies by De Meuter & Laga (1970), Hooyberghs & De Meuter (1972) and Laga & De Meuter (1972) led to the formal definition of the upper Miocene Diest Formation (De Meuter & Laga, 1976).

The bulk of the deposits constituting the Diest Formation are

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informally called the “Diest sand”. Near the base of the Diest Formation, two medium-fine to fine-grained sandy members have locally been distinguished: the Deurne Member and Dessel Member (Fig. 3). In contrast to the bulk of the formation, these sub-units do contain calcareous fossils at many locations.

Glibert & de Heinzelin (1955a,b) described the Deurne Sand sub-unit. It contains many calcareous macrofossils, was formerly named “sables à Terebratula perforata” by Nyst (1861), and it only occurs just east and NE of the city of Antwerpen. The unit consists of grey-green, medium-fine, very slightly clayey, glauconiferous sand, locally very rich in nests of Bryozoa, Brachiopoda and Ditrupa, with white, glauconite-rimmed trace fossils, and a base gravel with small rounded flint pebbles, bone fragments and shark teeth.

The presence of internal clasts of Terebratula brachiopods in the limonitic sandstones of the Diest Sand in the Hageland and Kempen regions allowed correlating the Diest Sand and Deurne Sand (Laga

& Louwye, 2006). Another sub-unit, the Dessel Sand, was identified in cored wells at Dessel (Laga & De Meuter, 1972) as the lower, fine- grained glauconiferous sand, containing foraminifera and organic- walled microfossils, of the same Diest Formation. The Dessel Sand appears to occur throughout most of the Kempen area, as the lower part near the base of the Diest Formation, and was called “sables fins du Diestien” by Gulinck et al. (1963).

The first geological maps of Belgium (1896, 1929) considered another glauconiferous sand unit, the “Kasterlee Sand” and its (quasi) lateral equivalent, the “Kattendijk Sand” (then known as

“sables à Isocardia cor”), as part of the Diest Formation. They were often referred to as the “Diestien supérieur”. They were considered as the regressive facies of the Diestien (Tavernier

& de Heinzelin, 1962). In some outcrops, the Kasterlee Sand unit contains reworked Diest Sand near its base (Fobe, 1995;

Verhaegen et al., 2014). Kasterlee Sand and Kattendijk Sand were later shown to represent two successive marine (or marginal marine) sedimentary deposits postdating the Diest Formation: the uppermost Miocene Kasterlee Formation (Louwye et al., 2007;

Vandenberghe et al., 2020, this volume), and the lower Pliocene Kattendijk Formation (Louwye et al., 2007).

3. Extent, correlatives and age of the Diest Formation The Diest Formation forms a depositional unit of varying thickness in the provinces of Vlaams-Brabant, Antwerpen and in the northern part of Limburg (Fig. 1b). In the NE, the thickness of the formation may reach up to 200 metres inside the Roer Valley Graben (RVG). The thickness decreases to the SW. Defying this general trend, in Hageland and Zuiderkempen, important thickness variations are related to incisions at the base. To the north, the formation is covered by younger Neogene deposits (Fig. 1b). In most of its occurrence area the Diest Formation is overlain by the Kasterlee Formation (Vandenberghe et al., 2020, this volume).

The Diest Formation is a marine deposit connected to the southern North Sea Basin, and more particularly to the embayment that during the Miocene occupied the Lower Rhine Graben. In this paper, the term “open sea” refers to this embayment as well as the North Sea towards the north. In and west of the Hageland area, the Diest Formation reaches further inland with respect to the occurrence of the older, underlying Oligocene and Neogene deposits (Fig. 1a). Here it fills a confined and incised basin whose long axis crosses at an angle the strikes of the older strata. This axis is directed WSW-ENE (about N60°E; Fig. 1b), and as it turns up repeatedly in relation with the Hageland Diest sand, it is named its “principal direction” in this article.

Figure 1. Location and outcrop maps. a) Outcrop map of Northern Belgium (DOV, 2019).

Thin Quaternary cover and locally thick Quaternary marginal marine, estuarine and fluvial sediments have been stripped.

The outcrop area of the Diest Formation (F.) and the underlying Berchem F. and Bolderberg F. as well as the overlying Kasterlee F. + Kattendijk F.

and Lillo F. + Poederlee F. has been labelled. Faults affecting the Diest F. have been added.

b) Diest F. outcrop and subcrop area (subcrop covered by Kasterlee F.) in NE Belgium.

Formation contours from G3Dv3 (Deckers et al., 2019). Location names cited in the text.

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Figure 2. Formerly used stratigraphic names, after De Meuter & Laga (1976, table 1, p. 140).

Name used in this

publication GSB code DOV code +

link to database Name used in this

publication GSB code DOV code +

link to database

Beerse URS 017W0354 2022-167 ON-Dessel-5 031W0370 ON-Dessel-5

Gasthuisberg - VLA17-4.1-001-TO1 Opitter seismometer 048E0294 BGD048e0294

Geel 046W0388 B/1-1100 Pidpa Oostmalle 029E0249 kb16d29e-B276

Herselt 060E0289 B/1-1115a Pidpa Poederlee 030W0300 kb16d30w-B315

Hoogstraten 007E0206 kb8d7e-B47 Pidpa Retie 031W0243 kb17d31w-B228

Lommel 032W0409 kb17d32w-B379 Rees 017E0399 kb8d17e-B495

Maaseik /Jagersborg 049W0220 kb18d49w-B220 Rijkevorsel 016E0153 kb8d16e-B37

Mol Belchim 031w0221 kb17d31w-B212 Scherpenheuvel 075E0340 kb24d75e-B344

Mol peilput 031w0237 B/1-0158 Veerle 060E0215A kb24d60e-B219

Neeroeteren 064W0234 kb26d64w-B242 Weelde 008E0133 kb8d8e-B26

ON Mol 2 031E0440 ON-Mol-2B Wijshagen 048W0180 kb18d48w-B181

ON-Dessel-2 031W0338 kb17d31w-B299 Zichem 076W0329 BGD076w0329

Table 1. List of boreholes referred to in this publication. Location: https://www.dov.vlaanderen.be/data/opdracht/2020-021774.

Currently used older names

LEGEND

Geological Map

(1896)

“Stages”

LEGEND

Geological Map

(1929)

“Stages”

STRATIGRAPHICAL TABLE

de Heinzelin, 1955 Tavernier & de Heinzelin, 1962

‘Sable gris glauconifère à Iso- cardia cor’ or ‘Sable à Ditrupa’

‘Sables et grès de Diest à Terebratula perforata’

or ‘Sable à Hétérocètes’

‘Sable noir d’Anvers à Pectunculus pilosus’

‘Sable argileux d’Edeghem à Panopea menardi’

‘Sables à Fusus (Chrysodomus) contrarius (or Neptunea c.)’

‘Falun blanchâtre’

PLIOCENEUPPER MIOCENE PLIOCENE PLIOCENE

MIOCENE MIOCENE

LOWERUPPER UPPER

MIDDLEMIDDLE MIDDLE

Scaldisien

Deurnien (= Diestien) Diestien

Bolderien

Bolderien Anversien

(= Antwerpiaan) Anversien Houthalenien Diestien

Sables de Kallo Sables de Luchtbal Sables de Kattendijk Sables de Diest Sables de Deurne

Sables d’Anvers Sables d’Edegem Sables d’Houthalen

Antwerpen area

Antwerpse KempenHageland areaLimburgse Kempen Roer Valley Graben

(Dutch part) Roer Valley Graben (German part)

MioceneLate Early Pliocene

Middle Miocene

Early Miocene

KASTERLEE FORMATION KATTENDIJK FORMATION no deposits

no deposits

no deposits no deposits

(eroded)

no deposits (eroded)

DIEST FORMATION

BERCHEM FORMA- TION

BOLDER- FORMATIONBERG

DIESSEN FORMATION

VELDHOVEN FORMATION GROOTE HEIDE

FORMATION

KIESELOOLITE FORMATION

Vrijherenberg Member Heksenberg

Member Kakert Member Someren Member Deurne Member

Kiel Member Edegem Member

Houthalen Member GenkMember

Opgrimbie facies

Kempen Diest sand

Hageland Diest sand

Dessel Member Hageland Dessel sand

Antwerpen Member

Zonderschot Member

?

Goirle Mbr.

Tilburg Mbr.

OOSTERHOUT FM.

INDEN FORMATION

VILLE FORMATION

FORMATIONKÖLN Frimmersdorf

Sand Neurath

Sand

Burdigalian Aquitanian Langhian Serra- vallian Tortonian Messinian Zanclean

Series Stage

EMU MMU

SAVIAN U.

LMU

lower upper

?lithofacies “D4”

?

Figure 3. Present-day lithostratigraphic names in their geographic context. Based on Vandenberghe et al. (2014, table 1) and Munsterman et al. (2020, fig. 8), with Hageland and Kempen Diest sand defined in this publication added. Existing formal names are in bold. EMU = Early Miocene Unconformity;

MMU = Mid Miocene Unconformity; LMU = Late Miocene Unconformity.

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The Diest Formation extends northward into the subsurface of the Kempen and the neighbouring area in The Netherlands. A small isolated remnant of the Diest Formation occurs just NW of Brussels. Another series of outliers constitute an up to 25 m thick unit found in the Flemish Hills, between Pottelberg (Flobecq) in the east and Cassel (N. France) in the west (Fig. 1a) although the stratigraphic position of this unit as part of the Diest Formation is disputed (Houthuys, 2014).

The Diest Formation was deposited during the late Miocene Tortonian (11.6–7.2 Ma) and earliest Messinian (7.2–5.3 Ma) (Louwye et al., 1999; Louwye & Laga, 2008). Organic walled microfossils could only be retrieved in the north and NE of the area of its occurrence. There, an overall progradation from SE to NW could be established, based on micropalaeontology (Louwye et al., 1999; Vandenberghe et al., 2014; King, 2016; Deckers &

Louwye, 2020).

In the subsurface of The Netherlands, the Diest Formation is included in the (former) Breda Formation, the latter containing all glauconiferous sand, clayey sand and clay deposited in The Netherlands during the Miocene (Van Adrichem Boogaert &

Kouwe, 1993–1997). The Diest Formation is chiefly part of the Tortonian and thus correlates only to the upper part of the Breda Formation. The Belgian-Dutch joint project “H3O – De Kempen” (Vernes et al., 2018, p. 183) modelled the “extended Diest Formation” as the Belgian Diest Formation which was correlated to the sandy, prograding middle and upper units of the Breda Formation, such as expressed in seismic lines, and defined at the base by the Mid Miocene Unconformity (MMU). The base of the extended Diest Formation was identified as a well traceable, strong impedance contrast.

Using lithological characteristics, large-scale depositional architecture revealed by seismic profiles and new biostratigraphic analyses, Munsterman et al. (2020) proposed to redefine the deposits of the Breda Formation. In this revision, the upper part of the Breda Formation that correlates to the Belgian Diest Formation is singled out as the new Diessen Formation (Munsterman et al., 2020). The Diessen Formation attains a maximum thickness of 500 m in the centre of the RVG in an area about 20 km north of Eindhoven. It thins towards the SW to 220 m inside the RVG and 125 m outside the RVG. The Diessen Formation is bounded below by a surface identified as the MMU. It is expressed as a depositional hiatus surface at least spanning the earliest Tortonian.

The top surface is an erosional truncation correlated to the Late Miocene Unconformity (LMU), and can be dated in the area at the Miocene to Pliocene transition. The correlation scheme in Munsterman et al. (2020) does not mention the Belgian Kasterlee Formation, but when biostratigraphic data are taken into account (Vandenberghe et al., 2020, this volume), its correlative deposits must be included in the Diessen Formation.

4. Lithology

Especially in the south and SE part of its extent the base of the formation often shows a gravel of rounded flint pebbles, usually without fossils. At locations where erosional incisions characterize the base of the Diest Formation, the pebbles are reworked from older deposits. In case fossils do occur in the basal gravel, they are a product of reworking. At the southern edge of the outcrops of Kesselberg near Leuven, two thin pebble layers are found inserted in the formation a few metres above its pebbly base. This demonstrates the close link between incision, erosion of older strata and incorporation of reworked elements in the lower part of the Diest Formation. A well-developed base gravel is also found near the city of Antwerpen; it is often rich in fossils.

A basal gravel and/or a level with coarse quartz grains has also been reported in the Antwerpen Province (Louwye et al., 2007;

Vandenberghe et al., 2014). It becomes more discontinuous going northward and eventually disappears.

The Diest Formation is dominated by poorly sorted, very glauconiferous quartz sands. Sometimes the formation is slightly clayey, locally with thick mud drapes or layered bundles of thin mud drapes associated with the bottomset part of large-scale cross- beds. The median and modal grain size is mostly above 250–300 µm and locally approaches 500 µm (Gullentops, 1963; Wouters

& Schiltz, 2012; Adriaens, 2015; Verhaegen, 2020, this volume).

In its lower part, especially in the Kempen, finer grain sizes

between 150 and 200 µm are found. Moderate to poor sorting is characteristic for the Diest Formation. Finer and coarser fractions are often represented in about equal amounts. Many samples contain a subpopulation of coarse, 0.5 to 2 mm, subangular or angular quartz grains. This fraction may also contain fragments of flint and other rocks. Cored samples are difficult to retrieve from the loose medium to coarse sand. The vertical grain-size profile is either (in the case of medium to coarse sand) stable, or shows a coarsening upwards trend. Secondary cyclic grain-size variations often characterize the profile. A comprehensive overview of the spatial and vertical grain-size variations throughout the area of the Diest Formation is still lacking.

The Diest Formation is highly glauconiferous: on average, glauconite pellets constitute between 35 and 40% of the total mass. Locally, values up to 50 and 60% are not exceptional. Lower glauconite values of about 25% occur in the Dessel Member (Vandenberghe et al., 2014). Glauconite pellets are slightly finer sized than the quartz grains in the same sample, indicating, as they are also slightly heavier, that they have been transported along with the quartz grains. The age determined using K/Ar dating on glauconite grains is systematically older than the age determined using micropalaeontology (Vandenberghe et al., 2014). This supports the observation that glauconite pellets in the Diest Formation are reworked.

Samples from the Diest Formation taken in medium to coarse sand may contain a minor clay fraction. It consists mostly of interstratified glauconite/smectite and represents an abrasion or disintegration product of the pelletal glauconite (Adriaens et al., 2014; Adriaens & Vandenberghe, 2020, this volume). Clay can also be present in mud drapes, or, where the deposit consists of fine sand, as dispersed clay. This is mostly detrital clay with a high smectite content (Adriaens & Vandenberghe, 2020, this volume).

The heavy mineral assemblage of the Diest Formation shows mixing of a northern, marine dominated provenance (epidote, amphiboles and garnet) and a southern, continental provenance (tourmaline, staurolite, kyanite and andalusite) (Verhaegen et al., 2019). A mostly northern marine provenance can be observed in the area near Antwerpen and in the basal Dessel Member. Most of the Kempen Basin shows a mixed signal but in the south and SE, near the RVG, a strong southern continental provenance can be deduced (Verhaegen et al., 2019; Verhaegen, 2020, this volume).

The ratio andalusite/kyanite is highest in the Flemish Hills, and gets progressively lower for the Hageland, Limburgse Kempen, Antwerpse Kempen and finally Antwerpen area. This ratio appears to be a good provenance indicator whereby the increased presence of andalusite can be linked to a southern provenance and associated with reworking of Eocene sediments from Flanders, the Ardennes and/or the Paris Basin.

Due to the relatively elevated topographic position of many Diest Formation outcrops in Hageland and Zuiderkempen, quartz grains often have a limonite coating and glauconite pellets are partly oxidized and altered at the surface. Heavy mineral samples taken from outcrops in the Hageland hills or from shallow subcrops in the Limburgse Kempen area show compositional shifts towards weathered assemblages, which is diagnosed by the disappearance of garnet and an increase in ultra-stable components (Verhaegen et al., 2019). The outcrops are also completely devoid of calcium carbonates. Siderite (iron carbonate) is found (Laga, 1972; Adriaens 2015), as well as rare steinkerns. They demonstrate that the entire formation originally contained carbonate particles and fossil remains, such as are still found in many drillings in the Kempen subcrop area, especially in the Dessel Member in the lower part of the formation (Adriaens, 2015). The original carbonate content was probably not very high, as sedimentary and biogenic structures are often well preserved, which would not be the case if strong decalcification had taken place. In all outcrops, especially in Hageland, where parts of the formation are situated above the (palaeo)groundwater tables, either irregular or more or less planar zones are loosely to strongly cemented by iron hydroxides and oxides. They yielded local building stones, the well-known Hageland iron sandstone (Bos & Gullentops, 1990; Dreesen et al., 2010; De Clercq et al., 2014). They can only have formed after emersion, in oxygenated conditions, and when an initial relief was installed. In the outcrop area, thick ironstones are found primarily near the sides of hills, not in their core (Houthuys &

Matthijs, 2018).

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5. The basal surface of the Diest Formation

The basal surface of the Diest Formation is relatively well known from outcrop and numerous boreholes. These are mostly flush borings, but even in those lacking dense sampling or continuous borehole logging, the base is easily recognized as a change in grain size and glauconite content, also accompanied by a change in relative firmness of the deposits. Most formation interfaces in the Belgian Neogene are fairly regular and smooth and form more or less parallel surfaces. The base of the Diest Formation is however a complex and highly erosive surface. Erosion preceded the deposition of the Diest Formation everywhere outside the RVG as variable thicknesses of the underlying Berchem and Bolderberg Formations have been removed (Vandenberghe et al., 2014). Tens of metres-deep incised depressions in western Hageland are observed in outcrop, around Leuven and Aarschot.

Borehole and seismic survey evidence (De Batist & Versteeg, 1998) shows similar-sized depressions are also present in east Hageland and the neighbouring Zuiderkempen area.

The map of the basal surface shown in Vandenberghe et al.

(2014, fig. 2) was that of the G3D version 2 model (Matthijs et al., 2013). G3D version 3 (Deckers et al., 2019) (Fig. 4) maintained the structure of version 2 for the incised area of Hageland and the Zuiderkempen. It was constructed with the underlying assumption

that the deepest points reflect a drowned river valley system (Timothy Lanckacker, pers. comm., in Houthuys, 2014). In the Antwerpse Kempen, new insights from geophysical borehole logs resulted in a smoother basal surface where many of the small channels modelled in version 2 are now no longer present. This implies a smaller amount of incisions in the Berchem Formation (Deckers et al, 2019).

However, the valleys connecting the deepest points are positioned in areas without boreholes and are thus not directly supported by data. An alternative interpretation presented in Figure 5 considers the deeper points to be situated in enclosed basal troughs. The idea of enclosed elongate depressions has earlier been applied in the map published by Houbolt (1982, fig.

7) which was based on data of the Hageland area compiled by Van Calster (1960). The closed trough model is locally supported by seismic surveys, though a definitive choice for a “valley” or

“closed trough” model for the basal surface awaits new evidence from well-located boreholes or geophysical surveys.

The basal surface of the combined Berchem/Bolderberg Formation can be consulted on DOV (2020). In the Antwerpse Kempen, it shows a uniform 0.45% slope dipping to N35°E.

More to the south, between Brussels and Heist-op-den-Berg, the general, averaged slope is smaller: about 0.2%.

Figure 4. Basal surface of the Diest Formation (coloured surface; elevation range from -200 m in NE to +90 m in SW) like modelled in G3Dv3 (Deckers et al., 2019) (see contour lines, 10 m-interval).

Black dots are boreholes where the base was found.

Figure 5. Basal surface of the Diest Formation (coloured surface; elevation range from -200 m in NE to +90 m in SW) based on manual interpolation (see contour lines, 10 m-interval) of base elevation in boreholes (dots). Source of data: G3Dv2 (Matthijs et al., 2013); seismic surveys (De Batist & Versteeg, 1998; Jef Deckers, pers. comm., 2019).

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North of the major incised zone, and also to the SW between Brussels and Heist-op-den-Berg, the basal surface of the Diest Formation displays the same slope values and strikes as the base of the Bolderberg/Berchem Formation. Both formations are therefore parallel, but nevertheless, the contact is an unconformity, also in the part of the Diest basin where no major incisions are found at the base, as a depositional hiatus is present between the top of the Berchem Formation (Langhian, 16.0–13.8 Ma) and the base of the Diest Formation (Tortonian, 11.6–7.2 Ma) (Louwye et al., 2007; Vandenberghe et al., 2014).

At about the same Serravallian (13.8–11.6 Ma) age, a major depositional hiatus is found in the UK, Danish, German and Dutch sectors of the North Sea (Wong et al., 2007; Rasmussen

& Dybkjær, 2014) related to an important regional event, called the Mid Miocene Unconformity (MMU). It has been associated with tectonic basin subsidence in the North Sea (Rasmussen &

Dybkjær, 2014) and with regional vertical tectonic rearrangement in the southern North Sea Basin area (Vandenberghe et al., 2014).

In the RVG, Utescher et al. (2012) and Schäfer & Utescher (2014) identify the level of the MMU in the Frimmersdorf lignite around the Langhian–Serravallian transition. More distal in the RVG, the MMU is expressed as a depositional hiatus surface at least spanning the earliest Tortonian (Munsterman et al., 2020). Hence also in Belgium, the observed hiatus between the Berchem and Diest Formations is most likely related to the larger scale MMU forming event in the North Sea Basin.

The thickness map of the Diest Formation presented in Figure 6 was produced using the basal surface map of Figure 5 and a new top surface of the Diest Formation. This is a reconstruction, for in the outcrop area, the formation is truncated and incised by the present terrain surface. A smooth flat surface connecting the top planes of the highest Hageland hills was constructed and then joined with the basal surface of the Kattendijk/Kasterlee Formations in the north as available in the G3D model.

The composed top surface is relatively smooth with a uniform slope dipping north. The dip increases from 0.2% in the southern part to 0.3% in the Noorderkempen. Therefore, the base Kattendijk/Kasterlee surface is a weakly expressed angular unconformity. The even morphology of the surface suggests it is a marine ravinement surface.

The Diest thickness map (Fig. 6), ignoring the thickness maxima situated in the elongated basal incisions, shows a uniform increase in thickness towards N65°E, i.e. orthogonal to the RVG. The wedge shaped thickness distribution probably reflects differential subsidence during the deposition of the Diest Formation, with stronger subsidence near the RVG border faults.

Alternatively, or in addition to the previous, it may indicate

stronger uplift after deposition of the more proximal SW part of the basin, followed by a more substantial truncation there. The overlying marine deposits all have an erosional base, often with a gravel. A truncation can also be inferred from regional profiles (e.g. profiles MG/0/280 and PGL/74/105, Laga, 1976).

The deviating thickness areas (filled channels or troughs) are clearly perpendicular to the RVG margin faults. The most important erosion troughs are situated near the SE margin of the “Diest Basin”. In areas where the basal surface of the Diest Formation crops out, thick cross-beds are associated with the incised surface. Such exposures are in the flanks of long Hageland hills. Elsewhere, no clear relationship between incised basal depressions and the location of Hageland hills could be observed, apart from the fact that their long axes share the same “principal direction”.

The faults bordering the RVG were active during the deposition of the Diest Formation. East of Dessel and Mol, the thickness of the formation abruptly increases from ca. 100 m west to ca. 150 m east of the faults there.

6. Internal variation inside the Diest Formation

Vandenberghe et al. (2014) suggested the Diest Formation might contain the sediments of two successive, separate Tortonian sedimentary sequences. The deposits associated with both sequences are in the present paper informally named the

“Hageland Diest sand” and the “Kempen Diest sand”, after their main area of occurrence.

The older Hageland Diest sand comprises the outcropping Diest Sand of Hageland and Zuiderkempen, which is mostly microfossil barren, along with the adjacent near-surface or subsurface units of the Diest Formation that contain dinoflagellate cysts of biochron DN8 (11 to 8.8 Ma, de Verteuil & Norris, 1996).

They are the Deurne Member, part of the Dessel Member (named hereafter the “DN8 Dessel sand” or the “lower Dessel Member”), and the lower part of the RVG Diest Sand and the easternmost outliers of the Diest Formation near Bree and Maaseik.

The younger Kempen Diest sand encompasses the upper part of the Dessel Member and the overlying Diest Sand found in the Noorderkempen and the RVG. These deposits yielded dinoflagellate biochrons DN9 and 10 (8.8 to 6 Ma).

The distinction between the two units relies mainly on differentiation in DN biozone (Louwye et al., 1999; Louwye &

Laga, 2008), K-Ar glauconite age (Vandenberghe et al., 2014), and heavy mineral associations (Hageland: larger portion of “southern” or “continental” provenance; Kempen: higher content of “marine” or “northern” source) (Verhaegen et al., 2019). The subdivision is however not (yet) firmly supported by

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KEMPEN

HAGELAND

Genk LEUVEN

Maaseik

HASSELT Turnhout

MECHELEN

BRUSSELS

EINDHOVEN

ANTWERPEN

MAASTRICHT ROER VALLEY

GRABEN

±

0 5 10 20 30 km

Thickness of Diest Formation [m]

0 - 10 11 - 20 21 - 30 31 - 40 41 - 50 51 - 60 61 - 70 71 - 80 81 - 90 91 - 100

101 - 110 111 - 120 121 - 130 131 - 140 141 - 150 151 - 160 161 - 170 171 - 180 181 - 190 191 - 200

Figure 6. Thickness map of the Diest Formation (coloured) made from G3D data supplemented with elevations of the top surface of hills with an interior of Diest Sand. The basal surface version of Fig. 5 was used.

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differences in quartz grain shape and size, glauconite content, or type of biogenic traces. There is a difference in sedimentary structures and large-scale sedimentary architecture, but also this topic needs more extensive evidence. More specifically, a clear surface separating the two sedimentary cycles has not yet been recognized.

7. Signature in geophysical logs

Geophysical logs of natural gamma ray radiation (GR) and electrical resistivity (RES) show a diverse response throughout the Diest Formation. Annex C in Vernes et al. (2018) contains interpreted profiles with logs for the H3O project area in the Belgian-Dutch border area, complemented to the north and the east in Munsterman et al. (2020). A comprehensive view to the south and towards the outcrop area is given here in Figure 7. As the Diest Formation is sandy throughout and often coarsening upwards, the Herselt log (GSB 060E0289 DOV B/1-1115a) represents the most representative response of a steadily upwards decreasing GR and increasing RES signal. The Zichem (GSB 076W0329 DOV BGD076w0329) and Scherpenheuvel (GSB 075E0340 DOV kb24d75e-B344) logs, situated near the Diest type locality, show a similar response in their top section, but they contain a thick basal part of constant values. In this interval, the sample descriptions mention sand and clayey sand

with clay laminae. The about 170 m thick section at Lommel (GSB 032W0409 DOV kb17d32w-B379) shows a signature that matches well that of the about 100 m thick sections of Mol-Dessel (peilput S15/A 1 Mol GSB 031W0237 DOV B/1- 0158, ON-Mol-2B GSB 031E0440 DOV ONMol-2B and ON- Dessel-5 GSB 031W0370 DOV ON-Dessel-5) and Weelde (GSB 008E0133 DOV kb8d8e-B26). Near Mol and Dessel, GR shows a steady, though sometimes discontinuous, upward increase over most of the interval, while RES exhibits more or less the mirror image. North of Mol and Dessel, in the H3O project area, the base of the Diest Formation starts with a few metres of quickly upwards decreasing GR signal, followed by the same signature as at Mol. The latter would commonly be interpreted as a fining upwards sequence. The opposite is true: the Diest Sand has in most places a vertically constant or coarsening upwards profile.

The high RES values indicate a high porosity, in this case for the basal section possibly related to well-sorted fine sand. The GR behaviour probably reflects variation in glauconite pellet content:

the amounts increase upwards. At the current stage, there is no standard log pattern to represent the Diest Formation and also the lower and upper transitions have no characteristic or basin-wide recognizable signature. A closer look at more logs, in relation to biostratigraphic and lithologic data would be needed to allow sound geometric and genetic interpretations.

Figure 7. Selected GR and RES logs across the Diest Formation with interpretations taken from the description in the archives (this interpretation may vary according to author, e.g. “Dessel” in ON-Dessel-5 is thicker in Adriaens, 2015; possibly, in some interpretations “clayey top” is considered part of Diest Formation and in others of Kasterlee Formation). Red curves are long-normal (thick line) and short-normal (thin line) resistivity, green curves are natural gamma ray.